* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    background-color: #2c3e50;
}

.container {
    position: absolute;
    top: 0;
    left: 50%;
    transform: translateX(-50%);
}

.container .main {
    width: 100vw;
    background-color: #7f8c8d;
}

.container .main img {
    width: 100%;
}

.btn-top {
    position: fixed;
    bottom: 0;
    right: 0;
    width: 9vw;
    height: 9vw;
    opacity: 0.2;
    transition: 0.2s ease-in-out;
}

.btn-top:hover {
    opacity: 0.8;
}

.show-height {
    position: fixed;
    top: 0;
    left: 0;
    width: 20vw;
    height: 9vw;
    display: flex;
    justify-content: center;
    align-items: center;
    background-color: aliceblue;
    z-index: 4;
}